It has heretofore been known that a plastic container obtained by forming a resin composition containing a scale-like inorganic pigment (e.g., talc) dispersed therein exhibits pearl-like (pearl luster) appearance. That is, light is scattered and reflected in the container wall due to the scale-like pigment. The plastic container exhibiting the pearl-like appearance has a very high commercial value.
In recent years, however, it has been strongly urged to reutilize the resources and attempts have been made to recover the used containers such as the above polyester containers to reutilize them as the recycled resins for a variety of applications. However, the plastic container having the pearl-like appearance is not suited for being recycled. This is because the scale-like inorganic pigment dispersed in the resin for imparting the pearl-like appearance makes it difficult to maintain transparency in the recycled resin.
A patent document 1 is proposing a plastic container imparted with light shielding property by dispersing bubbles (foamed cells) in the container wall.
In the plastic container of the patent document 1, light scattered by the foamed cells distributed in the container wall works to impart light shielding property, and the plastic container is very suited for being recycled since it uses no coloring component such as pigment. However, the plastic container of the patent document 1 has not been imparted with such an aesthetic appearance as pearl-like tone and does not have such an appearance that may enhance commercial value.
Further, when the container is formed by using a resinous foamed material, inconvenience often occurs due to the foaming. That is, a container as represented by a bottle has a threaded portion formed in the mouth portion thereof and a cap can be fitted thereto by screw-fastening. However, if the mouth portion of the container is formed of the foamed material, the strength decreases due to the foam and the dimensional stability decreases in the mouth portion of the container. In particular, a decrease in the dimensional stability at the threaded portion causes a decrease in the sealing performance when the cap is fitted.
To avoid the above problem, patent documents 2 and 3 are disclosing containers of which the body portions and bottom portions are formed of a foamed material but of which the mouth portions are not foamed.    Patent document 1: JP-A-2003-26137    Patent document 2: JP-A-61-53021    Patent document 3: JP-A-10-329203
The patent document 2 forms a preform (parison) in which is entirely distributed a foaming agent such as a decomposable foaming agent like an azo dicarbonimide or sodium bicarbonate, or a freon-type or hydrocarbon-type organic solvent foaming agent, heats the preform so as to be foamed except a portion corresponding to the mouth portion of the container, and stretch blow molds the preform in this state to obtain a container having the mouth portion that is not foamed. According to this method, however, the foaming agent is distributed over the whole preform, and the foaming cannot be effectively suppressed at the mouth portion of the container; i.e., the mouth portion of the container is merely a lowly foamed region where the degree of foaming is low, still leaving room for improvement from the standpoint of suppressing a decrease in the strength of the container mouth portion (threaded portion) and suppressing a decrease in the dimensional stability.
The patent document 3 produces a bottle by melt-kneading various foaming agents and a thermoplastic resin, directly blow-molding a preform while melt-extruding the kneaded mixture, forming a foamed preform by cooling the preform, heating the foamed preform again to a predetermined temperature, and blow-molding the bottle by blowing the air of a predetermined pressure. This method is to produce a bottle through a two-step blow which is a combination of a direct blow and a cold parison blow, and has a feature in that a portion corresponding to the container mouth portion is not expanded at the time of forming the foamed preform and the mouth portion of the finally obtained bottle has not been foamed. Even with this method, however, the foaming agent has been distributed over the whole foamed preform which is an intermediate product like in the patent document 2, and the mouth portion of the container is merely a region that is lowly foamed as compared to the body portion or the bottom portion. Namely, the foaming cannot be completely suppressed in the mouth portion of the container, which is not satisfactory from the standpoint of suppressing a decrease in the strength of the container mouth portion (threaded portion) caused by the foaming and suppressing a decrease in the dimensional stability.
There has further been known a method of separately forming a portion corresponding to the body portion or the bottom portion and a portion corresponding to the mouth portion, and joining the two together after having been formed. According to this method, the body portion and the bottom portion of the container are selectively foamed yet leaving the mouth portion of the container unfoamed. With this method, however, the body portion, bottom portion and mouth portion must be separately formed. Therefore, the productivity is low and the cost of production becomes high. Namely, this method cannot be employed from an industrial point of view.